Method for producing solar cells and thin-film solar cell

ABSTRACT

The invention relates to a method for producing a thin-film solar cell, comprising an absorber layer and at least one transparent window electrode. According to the method, the window electrode is produced with a first metal-based thin film, which receives an anti-reflection treatment, at least on the side on which the light is incident. According to the invention, the solar cell also comprises at least one first highly light-refracting oxide or nitride layer between the absorber layer and the first metallic layer. This leads to a considerable improvement in the conductivity of the window electrode and at the same time, reduces the thickness compared to conductivity of the window electrode and at the same time, reduces the thickness compared to convention window electrodes which usually consists of zinc oxide made conductive by doping.

[0001] The invention relates to thin-film solar cells having thecharacteristics of the preamble of independent claim 1, as well as to aprocess for the manufacture of thin-film solar cells having thecharacteristics of the preamble of claim 10.

[0002] It is known that photovoltaic solar cells built on a supportcomprise a front or window electrode, an absorber layer and a rearelectrode. In general, and in what follows, the electrode through whoseplane the light to be converted to voltage or electrical powerrespectively penetrates into the absorber layer is referred to as thewindow electrode. The window electrode must therefore be as transparentas possible or must have high light transmission, in order that it doesnot needlessly reduce the efficiency of the solar cell. On the otherhand, the rear electrode provided on the other face of the absorberlayer can be relatively thick and opaque. It must be characterizedsubstantially by the lowest possible surface electrical resistance andgood adherence to the absorber layer and, as the case may be, to theunderlying layer. In most cases, the rear electrodes are manufacturedfrom molybdenum metal, which satisfies the foregoing conditions.

[0003] In the most widely used type of thin-film solar cells, the rearelectrode is disposed between a support, the underlying layer, and theabsorber layer; the transparent window electrode is disposed on the cellface situated opposite the underlying layer. Consequently, theunderlying layer likewise does not necessarily have to be transparent.It can be made of glass, ceramic, polymer films or even of metallicsheets.

[0004] In solar cells with an overlying layer, the window electrode isdisposed between the support, which in this case must also be highlytransparent and, as the case may be, poorly reflecting antireflective,and the absorber layer, such that the light reaches the absorber layerthrough the support and the window electrode. In this case, the rearelectrode situated opposite the support does not have to be transparent.

[0005] The absorber layer is most often made of a layer of chalcopyritewith additions of copper, indium and selenium (known as CIS absorberlayers), sometimes also with sulfur instead of selenium. Occasionallythe absorber layer is also doped with gallium (CIGS absorber layers).The absorber layer generally exhibits p-type conduction. To manufacturea pn junction, a buffer layer of material having n-type conduction isapplied in a thickness of less than 100 nm on the absorber layer havingp-type conduction. It is known from U.S. Pat. No. 4,611,091 that cadmiumsulfide (CdS) can be used as material for the buffer layer, with aconductive window of ZnO placed thereabove.

[0006] If zinc oxide (ZnO) or another transparent oxide is used asmaterial for the window electrode, this material, which is dielectric initself, must be deposited as a doped semiconductor. The conductivity isachieved by doping, with aluminum or boron among other substances. Onthe industrial scale, these window electrodes are most often depositedby sputtering (cathodic sputtering under a magnetic field) on thesurface of the absorber layer. However, layers with a thickness of 400nm and more then are needed in order to limit the surface resistance toa usable level. Consequently, however, the light transmission is reducedcompared with thinner layers. Another drawback of this process is thatthe sputtering parameters, in particular the oxygen partial pressure inthe reactive atmosphere of the sputtering chamber, can be variablyadjusted only in a very narrow range in order to obtain optimal results.Finally, the deposition of relatively thick ZnO layers is alsotime-consuming and costly because of a relatively low rate of coatingwith zinc metal in a reactive atmosphere. As an alternative techniquethere can be used ceramic targets, which are already composed of thedesired conductive zinc oxide. Nevertheless, there is no advantage asregards deposition rate.

[0007] It is certainly possible to produce a ZnO window electrode withresults that are also still usable from the optical viewpoint, bychemical vapor deposition (CVD), but even thicker layers up to 1500 nmmust be tolerated in order to obtain satisfactory conductivity by thisprocess, because the material density of the layers produced in this wayis inferior to that of layers deposited by sputtering.

[0008] It has also been observed that a relatively thin layer (such as100 nm) of dielectric ZnO between the absorber layer and the windowlayer of ZnO made conductive by doping increases the efficiency of thesolar cell, and that it also has a positive influence on the stabilityof the process.

[0009] An advantage of this configuration, however, is that the knownsolar cells with CIS absorber layers have an open-circuit potentialwhich depends on a difference of charge between the absorber layer withp-type conduction and the ZnO electrode made conductive by doping (withn-type conduction).

[0010] The object of the invention is to provide a process for theeconomic manufacture of solar cells with an improved window electrodeand to propose thin-film solar cells equipped with such windowelectrodes.

[0011] According to the invention, this objective is achieved by thecharacteristics of independent claim 10 as regards the process and bythe characteristics of claim 1 as regards the solar cells. Thecharacteristics of the secondary claims respectively dependent onindependent claims 10 and 1 disclose advantageous modifications of theseobjects.

[0012] Thus the window electrode can be manufactured with a thinmetal-base layer, which is treated to be antireflective by at least onehighly refractive oxide or nitride layer at least on the side on whichlight is incident.

[0013] With the use of a metallic layer or of a metal-base layerrespectively, the conductivity of the window electrode is generallyincreased. With the antireflective treatment at least on the side of themetallic layer on which light enters (or in other words on its surfaceopposite the absorber layer), it is ensured that the usable light alsopasses effectively through the electrode and is not reflected for themost part or at all at the surface of the metallic layer.

[0014] In principle, it is of little importance whether or not theantireflective layer itself is electrically conductive. It may bedeposited in a single layer or in a succession of layers, with the onlylimitations that, on the one hand, it must be sufficiently transparentand, on the other hand, it must adhere well to the metallic layer and bechemically compatible therewith.

[0015] According to one embodiment of the invention, at least one of thedielectric layers is composed of zinc oxide.

[0016] According to the invention, the metallic layer is composed ofsilver or silver alloy and the antireflective layer is a highlyrefractive oxide or nitride layer.

[0017] In one modification, a highly refractive layer or succession ofsuch layers is also disposed between the absorber layer and the metalliclayer of the window electrode. In principle, it is also of littleimportance whether or not this is electrically conductive, bearing inmind that it must in no case substantially limit the flow of currentbetween the absorber layer and the window electrode by its ohmicresistance.

[0018] In a particularly preferred embodiment, the window electrode isproduced in the form of a succession of layers comprising a dielectriclayer, a base layer of metal or alloy and another dielectric layer.

[0019] Without departing from the scope of the invention, the windowelectrode can comprise successively a first highly refractive layer, afirst metallic layer, a second highly refractive layer, a secondmetallic layer and the said antireflective layer.

[0020] The highly refractive and possibly dielectric layers can bedeposited in known manner in the form of oxides (ZnO, SnO₂, BiO_(x),TiO₂, Al₂O₃) or nitrides (AlN, Si₃N₄). The metallic layer is preferablycomposed of elements or alloys having high conductivity, such as Ag, Cu,Au, Al or alloys thereof.

[0021] The solar cell according to the invention can also comprise atleast one metallic layer and one highly refractive oxide or nitridelayer.

[0022] In a particular embodiment, the metallic layer of the windowelectrode, particularly a silver layer, has a thickness of less than 20nm, and the total thickness of the window electrode is less than 120 nm.

[0023] To protect the already terminated conductive thin layer fromoxidation during the deposition of a highly refractive or respectivelydielectric layer, it may be possible to provide a blocking layer based,for example, on NiCr, Ti, Al or Pb between the metal-base layer and thehighly refractive or respectively dielectric layer to be depositedthereafter.

[0024] In principle, successions of layers comprising two dielectriclayers with an intermediate metallic layer (functional layer) aregenerally known as thermal insulation layers for reduction of theemissivity of glazing units of buildings or automobiles. The dielectriclayers therein act as antireflective treatments of the intermediatemetallic layer by virtue of the difference in refractive index. Withoutthe dielectric layers, the metal-base layer would also reflect too muchof the visible light, and this is in no case desirable for automobileglazing units.

[0025] It is also known that a relationship exists between theelectrical conductivity and the thermal insulation effect of thefunctional layer, such that its infrared reflection is also high whenthe conductivity is high.

[0026] The use of such successions of layers that are already widelyemployed and manufactured in large dimensions as window electrodes onthe one hand offers the advantage of clearly more economical manufacturecompared with the conventional ZnO window electrodes, and on the otherhand the total thicknesses of these layers can be appreciably reduced byvirtue of the clearly lower surface resistance, of a metallic silverlayer, for example, compared with the 400 nm of ZnO necessaryheretofore.

[0027] The window element according to the invention can thereforecomprise a succession of a first highly refractive layer, a firstmetallic layer and a second highly refractive layer.

[0028] Thus the necessary surface resistance of R_(□)<10Ω_(□) can beobtained with a silver layer having a thickness of less than 20 nm,disposed between two dielectric layers having thicknesses of about 30 to50 nm. In this way a window electrode having a thickness of less than120 nm can therefore be achieved.

[0029] Without departing from the scope of the invention, the windowelement can also comprise the succession just described, to which thereis added a second metallic layer and the antireflective layer. In thisstack of five layers, the metallic layer can be composed of silver or asilver alloy.

[0030] The window electrode according to the invention can be used notonly with CIS thin-film solar cells, but also for solar cells producedby other thin-film technologies in models containing underlying layer oroverlying layer. Solar cells with amorphous silicon or with cadmiumtelluride as absorber layer can also be equipped with the windowelectrode discussed here. In another conceivable embodiment, the twoelectrode layers of a thin-film solar cell can be replaced by thetransparent electrode according to the invention.

[0031] It is also possible to produce a combination of the electrodecontaining a metallic layer with a thin layer of conductive oxide,interposed between the absorber and the metallic layer.

[0032] To produce an example, there was deposited on a CIS thin-filmsolar cell having glass/Mo/CIS/CdS structure a window electrode havingthe following structure: dielectric 1 ZnO about 50 nm blocking Ti about3 nm metal Ag about 15 nm dielectric 2 ZnO about 55 nm Si₃N₄ about 30 nm

[0033] in which the silicon nitride layer acts substantially asmechanical protection against damage (scratches). An additional effectmay be the decrease of diffusion of moisture into the absorber layer,which reduces the storage stability of unshielded solar cells andrespectively the climatic stability of stratified solar modules.

[0034] For a layer of this nature deposited on glass, there has beendetermined a surface resistance of R_(□)=8.5Ω_(□). On a solar cellprovided with this layer, there was measured a reflection ratio of 1.2%in the region of the visible spectrum. Compared with conventional windowelectrodes, which have a reflection ratio of about 8%, this layerstructure therefore has a reflection-reducing effect, and it thereforeimparts improved light transmission in the absorber layer.

[0035] The specimen was divided into four solar cells, on each of whicha metallic contact was formed by vapor deposition. For referencepurposes, ZnO:Al made conductive by aluminum doping was deposited as thewindow electrode on a solar cell having the same structure.

[0036] The following characteristic values were measured on these solarcells: Open-circuit Short-circuit Fill voltage current factor EfficiencyU_(oc) [mV] I_(sc) [mA/cm²] FF [%] η [%] Cell 1 522 24.1 66.5 8.4 Cell 2535 23.9 66.6 8.5 Cell 3 517 24.8 66.3 8.5 Cell 4 509 24.8 64.9 8.2Reference 610 31   77   14.6 

[0037] Certainly the values of cells 1 to 4 are poorer than thecomparison values measured with the reference cell, but for the firsttime proof has been found of the applicability of the principle of thesandwich window electrode. It is hoped that even better values can alsobe achieved by adequate optimization of the deposition process and ofthe layer thicknesses.

[0038] The comparatively low efficiency of solar cells containing thewindow electrode according to the invention is explained by the factthat the wavelength region in which the window electrode is transparent(300 nm to 900 nm) is narrower than the region of high spectralsensitivity of the absorber layer (about 300 nm to 1300 nm). In thisspecific case, the usable quantity of incident light in the wavelengthregion between about 900 nm and 1300 nm is therefore reflected by thewindow electrode.

[0039] Results comparable to those of the reference specimen can beexpected when the region of high transmission of the window electrodehas been better adapted to the spectral sensitivity of the absorber, orin other words after it is possible for the window electrode also to betransparent for wavelengths longer than 900 nm up to about 1300 nm.Possibilities for lowering the upper reflection threshold by influencingthe conductivity of the metallic layer are well known to those skilledin the art. The conductivity of the metallic layer decreases, however,when its transparency for longer wavelengths is increased.

1. A thin-film solar cell comprising an absorber layer, particularly ofthe CIS type, and at least one transparent window electrode disposed onthe side on which light is incident, the said electrode comprising atleast a first metal-base thin layer and at least one antireflectivelayer deposited on the side on which light is incident, situatedopposite the absorber layer, characterized in that it additionallycomprises, between the absorber layer and the metallic layer of thewindow electrode, at least one first highly refractive oxide or nitridelayer.
 2. A thin-film solar cell according to claim 1, characterized inthat at least one of the dielectric layers is composed of zinc oxide. 3.A thin-film solar cell according to claim 1, characterized in that themetallic layer is composed of silver or silver alloy and in that theantireflective layer is a highly refractive oxide or nitride layer.
 4. Athin-film solar cell according to any one of the preceding claims,characterized in that the window electrode is formed by a succession oflayers comprising at least one dielectric layer, the said metallic layerand another dielectric layer.
 5. A solar cell according to any one ofthe preceding claims, characterized in that the window electrodecomprises in succession the said first highly refractive layer, the saidfirst metallic layer, a second highly refractive layer, a secondmetallic layer and the said antireflective layer.
 6. A thin-film solarcell according to any one of the preceding claims, characterized in thatat least one of the highly refractive layers is composed of one of theoxides ZnO, SnO₂, BiO_(x), TiO₂, Al₂O₃ and/or one of the nitrides AlN,Si₃N_(4.)
 7. A thin-film solar cell according to any one of thepreceding claims, characterized in that it comprises a second electrodecomposed of at least one metallic layer and one highly refractive oxideor nitride layer.
 8. A thin-film solar cell according to any one of thepreceding claims, characterized in that the metallic layer of the windowelectrode, particularly a silver layer, has a thickness of less than 20nm, and the total thickness of the window electrode is less than 120 nm.9. A thin-film solar cell according to any one of the preceding claims,characterized in that a blocking layer is disposed between the metalliclayer and one of the highly refractive layers.
 10. A process formanufacture of a thin-film solar cell comprising an absorber layer aswell as at least one transparent window electrode disposed on the sideon which light is incident, with at least one metallic layer and oneantireflective layer applied on the side on which light is incident,characterized in that it is manufactured in such a way that at least onehighly refractive oxide or nitride layer is provided between theabsorber layer and the metallic layer of the window electrode.
 11. Aprocess according to claim 10, characterized in that the windowelectrode is formed by a succession of layers with one thin metal-baselayer between two highly refractive oxide or nitride layers.
 12. Aprocess according to claim 9, characterized in that the window electrodeis formed by a succession of a first conductive dielectric ortransparent layer, of the metal-base conductive layer and of anotherconductive dielectric or transparent layer.
 13. A process according toany one of the preceding claims, characterized in that the solar cellcomprises a second electrode also made with at least one thin metalliclayer and one highly refractive oxide or nitride layer.
 14. A processaccording to one of claims 9 to 11, characterized in that the solar cellis made with an absorber layer of chalcopyrite.